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Kinematics and Dynamics of the M 51-Type Galaxy Pair NGC 3893/96 (KPG 302)

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Kinematics and Dynamics of the M 51-Type Galaxy Pair NGC 3893/96 (KPG 302) A&A 466, 847–854 (2007) Astronomy DOI: 10.1051/0004-6361:20077071 & c ESO 2007 Astrophysics Kinematics and dynamics of the M 51-type galaxy pair NGC 3893/96 (KPG 302) I. Fuentes-Carrera1,, M. Rosado2,P.Amram3,H.Salo4, and E. Laurikainen4 1 Instituto de Astronomía, Geofísica e Ciencias Atmosféricas, Universidade de São Paulo, Rua do Matão 1226-Cidade Universitária, 05508-900 São Paulo SP, Brazil e-mail: [email protected], [email protected] 2 Instituto de Astronomía, Universidad Nacional Autónoma de México (UNAM), Apdo. Postal 70-264, 04510, México, D.F., México e-mail: [email protected] 3 Laboratoire d’Astrophysique de Marseille, 2 place Le Verrier, Marseille Cedex 4, France e-mail: [email protected] 4 Department of Physical Sciences, Division of Astronomy, University of Oulu, 90570 Oulu, Finland e-mail: [email protected],[email protected] Received 9 January 2007 / Accepted 23 January 2007 ABSTRACT Aims. We study the kinematics and dynamics of the M 51-type interacting galaxy pair KPG 302 (NGC 3893/96). We analyze the perturbations induced by the encounter on each member of the pair, as well as the distribution of the dark matter (DM) halo of the main galaxy in order to explore possible differences between DM halos of “isolated” galaxies and those of galaxies belonging to a pair. Methods. The velocity field of each galaxy was obtained using scanning Fabry-Perot interferometry. A two-dimensional kinematic and dynamical analysis of each galaxy and the pair as a whole are done emphasizing the contribution of circular and non-circular velocities. Non-circular motions can be traced on the rotation curves of each galaxy allowing us to differentiate between motions associated to particular features and motions that reflect the global mass distribution of the galaxy. For the main galaxy of the pair, NGC 3893, optical kinematic information is complemented with HI observations from the literature to build a multi-wavelength rotation curve. We try to fit this curve with a mass-distribution model using different DM halos. Results. Non-circular motions are detected on the velocity fields of both galaxies. These motions can be associated to perturbations due to the encounter and, in the case of the main galaxy, to the presence of a structure such as spiral arms. The location of the corotation radius of this galaxy is also explored. We find that the multi-wavelength rotation curve of NGC 3893, “cleaned” from the effect of non-circular motions, cannot be fitted whether by a pseudo-isothermal or by a NFW DM halo. Key words. galaxies: interactions – galaxies: kinematics and dynamics – galaxies: individual: NGC 3893, NGC 3896 – galaxies: spiral – galaxies: halos 1. Introduction the classical method for studying mass distribution, see Blais- ff Ouellette et al. (2001), and references therein. The RCs also al- The di erence between the mass distribution implied by the lu- low us to determine the maximum rotation velocity of a galaxy minosity of a disk galaxy and the distribution of mass implied ff and thus infer the total mass within a certain radius using meth- by the rotation velocities o ers strong evidence that disk galax- ods such as that of Lequeux (1983). Nevertheless, care must be ies are embedded in extended halos of dark matter (Sofue & taken when using kinematic information from interacting galax- Rubin 2001, and references therein). Detailed knowledge of dark ies, since they are subject to kinematical perturbations that may matter (DM) halos around galaxies holds important clues to the affect the correct determination of an RC that actually traces the physics of galaxy formation and evolution and is an essential in- global mass distribution of the galaxy. For this reason, 3D spec- gredient for any model aiming to link the observable Universe troscopy observations are required to separate circular from non- with cosmological theories. In practice, realistic DM halos are circular motions in the velocity field of a galaxy and its rotation neither static nor spherically symmetric (Knebe et al. 2004) and curve as shown in Fuentes-Carrera et al. (2004). it is still unknown if their structure and distribution is intrinsi- cally related to the environment of their galaxies. The question In this work, we present scanning Fabry-Perot observa- / remains as to whether an intrinsic difference exists between the tions of the M 51-type interacting galaxy pair NGC 3893 96 DM halo of an “isolated” galaxy, the DM halo of a galaxy be- (KPG 302). Section 2 presents the scanning Fabry-Perot (FP) longing to a pair or that of a galaxy that is part of a larger group observations and data reductions. Section 3 introduces the pair / such as a compact group or a cluster. of galaxies KPG 302 (NGC 3893 96). In Sect. 4 we present In this sense, rotation curves (RCs) are a powerful tool the kinematic information derived from the F-P observations. for studying the distribution of matter (both baryonic and non- Section 5 presents the dynamical analysis of both galaxies, mass baryonic) in interacting disk galaxies. For a description of estimates and the mass distribution for NGC 3893. The discus- sion and conclusions are presented in Sects. 6 and 7, respec- Presently at the Observatoire de Paris-Meudon. tively. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20077071 848 I. Fuentes-Carrera et al.: Galaxy pair KPG 302 2. Observations and data reductions Observations of NGC 3893/96 (KPG 302) were done at the 2.1 m telescope at the OAN-SPM (México) using the scanning Fabry-Perot interferometer PUMA (Rosado et al. 1995). PUMA is a focal reducer built at the Instituto de Astronomía-UNAM used to make direct images and Fabry-Perot (FP) interferometry of extended emission sources (field of view 10). The FP used is an ET-50 (Queensgate Instruments) with a servostabilization system having a free spectral range of 19.95 Å (912 km s−1)at Hα. Its finesse (∼24) leads to a sampling spectral resolution of 0.41 Å (19.0 km s−1) achieved by scanning the interferometer free spectral range through 48 different channels. A 1024 × 1024 Tektronix CCD detector with a resolution of 0.58/pixel was used. We used a 2 × 2 binning to enhance the signal. The fi- nal spatial sampling equals 1.16/pixel. To isolate the redshifted Hα (λat rest = 6562.73 Å) emission of the galaxies, we used an interference filter centered at 6584 Å with an FWHM of 10 Å. / To average the sky variations during the exposure, we got two Fig. 1. a) Direct B image of NGC 3893 96 (KPG 302) from “The data cubes with an exposure time of 48 min each (60 s per chan- Carnegie Atlas of Galaxies. Volume II” (Sandage & Bedke 1994). b) Monochromatic Hα (continuum subtracted) image of the pair ob- nel). These data cubes were co-added leading to a total exposure tained from the scanning Fabry-Perot interferometer PUMA data cubes. time of 96 minutes. For the calibration, we used a H lamp whose Upper panel: Optical image with HI isophotes superposed. Image 6562.78 Å line was close to the redshifted nebular wavelength. taken from Verheijen & Sancisi (2001) in “An HI Rogues Gallery” Two calibration cubes were obtained at the beginning and at the (http://www.nrao.edu/astrores/HIrogues/webGallery/ end of the galaxy observation to check the metrology. RoguesGallery06.html) Data reduction and analysis were done using mainly the ADHOCw1 and CIGALE softwares (LeCoarer et al. 1993). particular concentration toward the center. Given the isolation Standard corrections (cosmic rays removal, bias subtraction, criteria used in the KPG and the nature of the cluster, it is pos- flat-fielding, etc.) were done on each cube. Once the object sible that the DM halo (or halos) of the galaxies in the pair are cubes were co-added, the night sky continuum and 6577.3 Å isolated from those of the cluster. OH sky line were subtracted. A spectral Gaussian smoothing = −1 NGC 3893 is a grand-design spiral similar to NGC 5194 (σ 57 km s ) was also performed. Once the spectral smooth- in M 51 (Fig. 1a). It has been classified as SABc in LEDA ing was done, the calibration in wavelength was fixed for each database, and as SAB(rs)c in the NED2 database and in the profile at each pixel using the calibration cube. The Fabry-Perot RC3 (de Vaucouleurs et al. 1991). However, Hernández-Toledo scanning process allows us to obtain a flux value at pixel level for & Puerari (2001) classify it as a non-barred galaxy without any each of the 48 scanning steps. The intensity profile found along inner ring. The HI observations (Verheijen & Sancisi 2001) show the scanning process contains information about the monochro- that this galaxy is slightly warped in its outer parts – both in its matic emission (Hα) and the continuum emission of the object. HI distribution and its HI kinematics. Its companion, NGC 3896, The continuum image computation was done considering the appears to be an intermediate type galaxy between and S0 and mean of the 3 lowest intensities of the 48 channels cube. For a spiral, which also has a bar. It shows extended Hα emission. the monochromatic image, the Hα line intensity was obtained NGC 3893 shows no color excess, whereas NGC 3896 has a by integrating the monochromatic profile in each pixel. The ve- predominantly blue B − V color in the central parts of the galaxy locity maps were computed using the barycenter of the Hα pro- ffi (Laurikainen et al.
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